This invention relates to charged particle beam optics and, more particularly, to an imaging lens which provides the capability to rotate an image without substantially affecting the focus or magnification of the image.
Electron beam exposure systems are used commercially for selectively irradiating a resist coated workpiece, which can be a mask plate or a semiconductor wafer. A finely focused electron beam is deflected over the surface of the workpiece to define a prescribed pattern. The electron beam is controlled in a highly accurate, high speed manner to expose microminiature patterns in the electron resist material. Various approaches have been taken in controlling the electron beam. A small circular spot beam can be raster scanned over the entire surface of the workpiece and turned on or off to produce the desired pattern. Alternatively, the spot beam can be directed to desired pattern areas and scanned over those pattern areas in a vector scanning approach. Either approach is relatively slow, since the area covered by the spot beam at any instant is extremely small.
In another approach, the electron beam is shaped into a rectangle of variable size and shape. The shaped beam can be utilized in a step-and-repeat mode to expose the desired pattern. In a preferred shaped electron beam exposure technique, an elongated rectangular beam, or line beam, is scanned in a direction perpendicular to its long dimension. As the beam is scanned, the length of the line is varied to correspond to the desired pattern. This approach permits patterns of almost any shape to be exposed in a single operation. Shaped beam exposure systems provide generally higher operating speeds than spot beam systems and have gained favor in systems for direct writing on wafers.
In producing a shaped beam, a two-aperture shaping system is frequently employed. An image of a first aperture having two orthogonal edges is focused on a second square aperture by an imaging lens. A shaping deflector dynamically positions the image of the first aperture relative to the second aperture so that the beam passing through the second aperture is shaped to the desired cross-sectional length and width.
Present day microlithography systems require extreme accuracy. Minimum feature sizes are on the order of one micron or less and feature accuracies are usually less than one-tenth micron. Exposure variations must be less than 3%. With the two-aperture beam shaping technique described above, the final beam image is defined by two edges of the first aperture and two edges of the second aperture. Therefore, any rotation of the image of the first aperture relative to the second aperture produces a beam which is trapezoidal rather than rectangular. Image rotation can result from mechanical misalignment of the apertures or from beam rotation introduced by magnetic lenses and deflectors. Furthermore, as the magnetic field of the imaging lens is varied in order to focus the image of the first aperture onto the second aperture, undesired rotation is introduced. Typically, beam rotations have been made by mechanically adjustable apertures which provide the desired rotation but are difficult to use and add complexity to the construction of the electron beam column.
It is desirable to incorporate into the imaging lens the capability of rotating the image of the first aperture by application of an electrical signal. The rotational adjustment of the beam should not substantially change either the focus or the magnification of the image. Conversely, the imaging lens should be capable of focusing the image of the first aperture on the second aperture without rotation. Furthermore, as the beam is positioned by the shaping deflector, the distortion introduced by the imaging lens must be within prescribed limits. Existing magnetic lens cannot meet all of the above requirements. With a single gap lens, the beam is defocused when a rotational variation is made. With a doublet, or two-gap lens, the beam stays in focus as a rotational adjustment is made but the magnification is altered.
It is a general object of the present invention to provide novel magnetic lenses for charged particle beams.
It is another object of the present invention to provide magnetic lenses which can introduce variable beam rotation without substantially changing the focus and magnification of the image.
It is yet another object of the present invention to provide magnetic lenses in which the focus can be varied without substantially changing the beam rotation introduced by the lens.
It is still another object of the present invention to provide magnetic lenses with independently variable focus and rotation adjustments wherein distortion is within prescribed limits.